Laser-induced fluorescence (LIF) has been investigated recently as a method to discriminate tumors from normal tissues. LIF techniques have also been used to characterize pre-malignant colorectal lesions and to distinguish adenomatone polyps from normal colon tissue and hyperplastic polyps.
Others have investigated the use of LIF to distinguish adenomatous tissue from normal colon tissue in vivo. In these investigations, a laser such as a pulsed nitrogen laser-pumped dye laser system (370 nm) was used as the excitation source. The sensitivity, specificity and predictive value for diagnostics of their technique towards adenomas were reported to be very good. Because only a small number of hyperplastic polyps were examined, it is unclear whether colonic neoplasia can be reliably identified, since it is not known whether the observed differences in fluorescence arise from compositional changes specific to dysplasia or simply from structural differences between polyps and the colon.
The LIF technique has also been used to distinguish adenomatous from normal colon tissue in vitro. In a study conducted by Kapalia et al. in 1990, endoscopically resected polyps were excited using a continuous wave (cw) helium-cadmium laser (325 nm) and the resulting fluorescence of these endoscopically resected polyps was measured with an optical-multichannel-analyzer system. They found that adenomatous polyps (51 of 51) could be reliably distinguished from normal colonic tissue (69 of 69) in vitro based on LIF scores from a stepwise multivariate linear regression (MVLR) analysis of their data. In addition, 15 of 16 hyperplastic polyps fell within the normal colonic tissue range, resulting in the ability to distinguish colonic neoplasia of resected tissue.
Schomacker et al., in 1992, also used a MVLR analysis method to distinguish neoplastic tissue from non-neoplastic tissue. Their data suggested that the LIF measurements sense changes in polyp morphology rather than changes in fluorplores specific to polyps, and it was this change in morphology that leads indirectly to discrimination of polyps. Schomacker concluded that the feasibility of discriminating groups of normal from dysplastic cells by LIF had not yet been demonstrated.
The above examples underscore the fact that, in spite of some specific successes, one of the major limitations of the LIF technique is its specificity. The laser used as the excitation source used under current conditions can yield high intensity but does not provide a selective tool for excitation.
Tissue fluorescence is a complex process arising from the superposition of the fluorescence of many chemical species in tissue. Although changes in fluorescence profiles have been reported by many researchers involved, these changes are often difficult to provide unique "spectral signatures" useful for unequivocal diagnostic purposes.
In addition to spectral specificity problems, current instrumentation for cancer diagnostics have serious limitations. A laser-based LIF instrument can use only fixed excitation whereas conventional spectrometers (non-laser devices) do not provide rapid synchronous luminescence (SL) scanning capabilities for useful clinical applications.
There is, therefore, a strong need to develop new or improved methods and instrumentation for sensitive as well as selective chemical analysis and biomedical diagnostics.